1. Field of the Invention
The present invention generally relates to a PWM (pulsewidth modulation) control type power supply apparatus. More specifically, the present invention is directed to a PWM-controlled power supply apparatus capable of eliminating modulation-frequency signal components (noises) from ground potentials at input/output sides thereof.
2. Description of the Related Art
In FIG. 1, there is shown a circuit diagram of a conventional PWM control power supply apparatus. Two diodes 2 and 3 are connected to a AC (alternating current) power source 1.
Capacitors 4 and 5 are also connected to the AC power source 1 so as to constitute an AC-to-DC converting circuit. In this case, a line of the AC power source 1 connected to a ground line is connected to a junction point series-connected between these capacitors 4 and 5. A main DC-to-AC inverter circuit 6 is connected to an output of the AC/DC converting circuit. The DC/AC inverter circuit 6 is arranged by both series-connected switching elements 61 and 62, and diodes cross-coupled with the respective switching elements. An output voltage of this inverter circuit 6 appears across a junction point "A" between these switching elements 61 and 62, and also another junction point "N" between these capacitors 4 and 5. This output voltage is applied via a filter 7 to a load 8. An PWM (pulsewidth modulation) controller 9 is connected to the switching elements 61 and 62 employed in the main inverter circuit 6 in the known pulsewidth modulation method.
In the above-described conventional PWM-controlled power supply apparatus, when the pulse-width modulation frequency is selected to be a high value, e.g., 20 KHz, high frequency (high modulation frequency) noises in a common mode may happen to occur which is negligible with respect to the ground potentials of the input/output terminals of such power supply apparatus. That is, while turning ON/OFF the switching elements 61 and 62, a ground potential appearing at the point "A" is alternately switched based upon voltage values of "VP" and "VN". As a result, charge/discharge currents to a stray capacitance "C.sub.0 " existing between this point "A" and the ground will flow through the ground line "N" in a pulsatory form, thereby resulting a voltage drop.
This stray capacitance "C.sub.0 " is necessarily produced between such a module type switching element and the major circuit arrangement, i.e., a housing of the PWM control type power supply apparatus. Precisely speaking, this stray capacitance "C.sub.0 " may be determined by a stray capacitance inherent to the switching element and another stray capacitance caused by the housing of the power supply apparatus, and may not be reduced to a predetermined value.
As previously described, the higher the modulation frequency of the DC/AC inverter 6 becomes with employment of a switching element having a high-speed switching characteristic, the greater the voltage variation velocity "dv/dt" at the point "A" becomes. As a result, the charge/discharge currents to the stray capacitance "C.sub.0 " analogously become large and thus the above-described voltage drop is no longer negligible.
Under such high modulation frequencies, high modulation-frequency noises (common mode noises) may be contained in both the input terminal and also the output terminal of the conventional PWM control power supply apparatus. Accordingly, no sufficient noise protection is taken in the above-described conventional PWM control type power supply apparatus when such a load having a highly noise-sensitive characteristic is connected thereto. Moreover, there is a risk that electronic appliances parallel-coupled to the input side of this conventional power supply apparatus may be erroneously operated due to such high modulation-frequency noises.